Polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) blends, products commercially available from SABIC Innovative Plastics, are an important class of polymeric materials for reasons of their excellent balance of properties such as low-temperature ductility, heat resistance, and outstanding aesthetics combined with ease of processing.
The polycarbonate portion of such a blend, however, is susceptible to degradation by acids and bases, especially under conditions of high heat and humidity. Degradation can cause undesirable color formation, loss of molecular weight, reduced chemical resistance or inferior mechanical properties, and generation of volatiles that can cause defects in surface appearance, for example, splay in molded parts.
These problems can be exacerbated by the addition of elastomeric impact modifiers that are commonly used to improve the toughness of polycarbonate compositions. In particular, an elastomer-containing impact modifier prepared by emulsion polymerization can decrease the stability of a polycarbonate. Specifically, compounds employed as aids in the emulsion polymerization of the impact modifier, during its preparation, can remain as residues within the impact modifier and can, thereby, promote transesterification or degradation of a polycarbonate. For example, U.S. Pat. No. 7,393,896 discloses that alkali metal salts of fatty acid emulsifiers, used during emulsion polymerization of an impact modifier to stabilize the emulsion, can later catalyze transesterification or degradation of a polycarbonate composition containing that impact modifier. In turn, the residual amount of emulsifiers remaining in the impact modifier can cause adverse effects, including inconsistent thermal stability of the polycarbonate composition. This can lead to problems during the molding of the polycarbonate composition, due to the variability in viscosity resulting from the catalytic transesterification or degradation of the polycarbonate component.
In view of the above, the use of emulsion-prepared elastomer-modified graft copolymers in polycarbonate compositions has been avoided, in some cases, in favor of bulk polymerized elastomer-modified graft copolymers, particularly when a stringent requirement for hydrostability exists such as exists with respect to molded parts used in the automotive industry. Bulk polymerized acrylonitrile-butadiene-styrene (“bulk ABS”) is essentially free of any surfactant. Accordingly, when higher hydrostability is desired, bulk ABS has been used in admixture with a small amount of sulfonate-based MBS to improve the impact resistance of a polycarbonate composition, as disclosed in EP 0900827 and U.S. Pat. No. 7,393,896 assigned to Sabic Innovative Plastics.
Because of the relatively lower elastomer content in bulk ABS, however, it usually cannot be used (or used alone) when higher impact resistance is desired, such as could otherwise be obtained by an emulsion-prepared ABS. An emulsion-prepared ABS can have elastomer levels as high as 60 wt. % or more.
As indicated above, replacing alkali metal carboxylate based surfactants, previously used in preparing elastomer-modified graft copolymers, with a sulfonate surfactant can improve the stability of a polycarbonate/ABS blend, as disclosed by U.S. Pat. No. 7,393,896. However, further improvement is still desired to meet stringent demands for stability required for certain molded parts.
In view of the above, an object of the present invention is to provide an impact-modified polycarbonate composition having improved resistance to polymer degradation, even under strenuous conditions. In particular, an improved balance of high impact resistance and excellent hydrostability and thermal stability is desired.